For instance, in Secrets of Strixhaven , play boosters have a 1 in 55 chance of having one (or 1.81%), in MH3 , a Special Guest showed up in one of every 15.5 collector boosters
Most of them are pretty fiddly and revolve around combat triggers, with Ink-Treader Nephilim leading the pack as the most interesting (and annoying) of the cycle
Bumbleflower on the battlefield, you make one of your opponents draw a card (Im sure you can see the politics of that coming into play already)
Exile Teferi's Protection
The only way to get rid of them is essentially through exiling them
And, of course, theres Kilo, Apogee Mind, one of the two face commanders of Edge of Eternities Counter Intelligence precon